The USAT 55 foot tank car, if operated on curves sufficiently greater than 10 foot diameter, may not suffer from wheel flange interference against the 906 coupler box.

Just a suggestion: If making a replica of the factory spacer, try making it about 0.030 inch thinner so the Kadee coupler aligns with the Kadee gauge. But check if the spacer and box can still be mounted OK.